+1. This is the definitive guide to class and class library design. These are the basic standards that every .Net developer should be familiar with. This is common ground so no matter what employer you go to, if they're a .Net development shop, then their standards are very likely to be very close to these guidelines.
–
DavidOct 29 '09 at 13:11

You need to provide a simple to use API, full documentation and worked examples as a minimum. If you can provide unit tests that would be a bonus.

Internally, you need to verify all inputs into your routines, as well as clearly document what configuration the user is expected to do. Solid exception handling is a given, and you should possibly consider some localisation support in there as well.